1. Field of the Invention
The present invention relates to the rolling of metal strips and, more particularly, to a technique for accurately controlling the temperature of a strip during the rolling process.
2. Description of the Prior Art
Sheet metal is produced by rolling slabs, bars or other relatively massive workpieces into elongate, thin strips. Although finish rolling often is done under room temperature conditions, the initial reduction of the workpiece from its bulk form is done at elevated temperatures in a facility known as a hot strip mill. In a hot strip mill the workpieces are heated in a reheat furnace to a temperature of around 2200 degrees Fahrenheit (.degree.F.). The reason the workpieces are heated to such an elevated temperature is that the temperature of the workpiece influences the resistance to deformation of the workpiece. That is, a hot workpiece has a lower resistance to deformation than a cold workpiece and, accordingly, requires less roll force before it will be deformed by a given amount than a cold workpiece of similar composition and dimensions. In short, deforming a workpiece maintained at an elevated temperature may be done easier and faster than one maintained at a lower temperature.
The temperature at which the rolling process is commenced is not maintained throughout the mill. As the strip passes from one stand of rolls to another, heat losses caused by radiation and/or strip-to-roll conduction reduce the temperature of the strip to about 1500.degree. F. to 1700.degree. F., depending in part upon the thickness of the strip. After the strip leaves the last mill stand, it must be cooled further prior to being coiled and banded. The Runout Table Cooling Patent describes a particularly effective technique for reducing the temperature of a hot rolled strip between the time the strip leaves the last mill stand and before it is coiled.
In addition to the obvious goal in a hot strip mill of reducing a workpiece to a desired finish thickness, it also is important that the temperature drop from the initial elevated temperature to the finish temperature and to the temperature at which the strip is coiled be controlled as much as possible. As explained more completely in the Runout Table Cooling Patent, metallurgical properties of hot rolled strip metal are dependent not only upon the composition of the metal, but also upon the temperatures at which the final thickness reductions take place, the temperature at which the strip is coiled, and the rate at which the temperature of the strip changes during the final cooling process. When steel is the material under consideration, the final thickness reductions normally take place above 1600.degree. F. and the strip is cooled on the runout tables to approximately 1200.degree. F. While finishing and coiling temperatures are of principal importance, the temperature at reductions preceding the final reduction may be important in achieving certain metallurgical properties. For these metallurgical grades, it is desirable to maintain constant temperature not only at the final rolling stands but at one or more of the preceding rolling stands. A secondary consideration relates to the control of strip flatness during rolling. In modern, computer-controlled hot strip mills, one objective of the reduction schedule is to assign reductions in successive stands which will produce roll separating forces and associated strip crowns which are compatible with good flatness. These strategies are well known and are described, for example, in "Automatic Shape Control-- Hoogoven's 88--In. Hot Strip Mill" by F. Hollander and A. G. Reinen, Iron and Steel Engineer, April, 1976. Strip flatness control will be improved where the rolling force at each stand is maintained more nearly constant throughout the strip length. This would of course require that intermediate as well as final rolling temperatures be held essentially constant. In short, by maintaining temperature at all stands essentially constant in the presence of variations in incoming strip temperature and variations in rolling speed, both metallurgical qualities and strip flatness can be enhanced.
A significant problem in maintaining the temperature of a metal strip at desired predetermined levels relates to "skid marks." Skid marks are sections of a strip at temperatures significantly below the average temperature of the strip, often by as much as 100.degree. F. Skid marks are caused because the workpieces are pushed through the reheat furnace on skids or other supports. The skids are water cooled and, thus, are at a lower temperature than the temperature of the rest of the furnace. Accordingly, small sections of the workpiece in direct contact with the skids will not be heated as much as other portions of the workpiece. The temperature deviation of the areas of the workpiece in contact with the skids is carried throughout the remainder of the rolling process, even though the great initial temperature deviation may be largely attenuated by the time the rolling process is complete. In any event, the existence of the skid marks causes a temperature variance in the strip along the length of the strip. This has made it difficult to control the temperature of all portions of the strip with a great deal of accuracy.
Another important consideration influencing the temperature of the strip is that of rolling speed. Modern high speed rolling mills thread the initial portion of the workpiece through the mill and coiler at a relatively low speed and accelerate rapidly to higher speeds where most of the rolling is done. All of the heat transfer phenomena in the rolling process are time dependent. Strip temperature loss through radiation and conduction to the work rolls is reduced at higher speeds, while energy input to the strip may be slightly increased due to strain-rate related increases in deformation resistance. At the same time, strip temperature on entering the finishing train may be decreasing due to radiation loss. Additionally, the cooling effect of interstand sprays is dependent upon strip speed not only because of the reduced cooling time at higher speeds, but also because of interaction of the closely spaced sprays comprising a group due to incomplete recovery of surface temperature on entering successive spray regions. These considerable variations make it very difficult to predict with any degree of accuracy the temperature which the strip will attain as it passes through the various mill stands.
On mills not equipped with interstand cooling sprays, control of strip finishing temperature has been achieved by adjustments to the finishing speed. The necessary adjustment has, in some cases, been precalculated to exactly compensate for the variation in temperature of the strip entering the first rolling stand. The temperature achieved in this manner can then be sensed by means of a pyrometer located immediately downstream of the last mill stand. If the temperature exiting the last mill stand is too high, the mill can be slowed down; if the temperature is too low, the mill can be accelerated. A major disadvantage with this method is that the maximum speed and, therefore, the production rate are determined by the temperature of the incoming strip. A second disadvantage is that the correction technique is very slow and large portions of the strip may be finished at incorrect temperature.
The potentially most effective technique to control the temperature of a strip as it is being rolled is to provide a number of individually controllable water sprays between adjacent mill stands. If the sprays are positioned above and below the strip and across the width of the strip, effective cooling of the strip can be accomplished. Consequently, higher rolling speeds are made possible and the temperature increases caused by the higher rolling speeds can be corrected through the use of water sprays.
The single most important problem with the water spray approach has been in properly sensing the temperature of the strip and thereafter controlling operation of the water sprays. Although a pyrometer positioned downstream of the last mill stand has been very effective as a monitoring device, presently available pyrometers and other temperature sensors have not been sufficiently accurate to permit a reliable indication of interstand strip temperature. Another consideration is that of transport lag, that is, the problem of sensing the temperature of the strip at one downstream location, providing an upstream temperature correction, and having to wait for the results of the temperature correction to become known to the downstream temperature sensor. Because of the stand spacing and total elongation between the first interstand spray and the downstream pyrometer, an error or a correction in strip temperature at the first interstand spray may not be evident until 300 or 400 feet of strip have passed the interstand spray location.
In an attempt to overcome the foregoing problems associated with water spray control, various prior art proposals have been made, all without much success. One of these proposals has been to calculate in advance a temperature profile for a strip of different thicknesses, rolling speeds, and so forth. As the mill accelerates to its desired rolling speed, water sprays are commenced at predetermined intervals. The sprays are activated first near the last mill stand and are activated sequentially in an upstream direction. During mill deceleration, the sprays are deactivated in the reverse sequence. A fundamental problem with this approach is that most of the cooling takes place toward the final mill stands. This means that the temperature corrections tend to be concentrated toward the end of the rolling process rather than distributed through the rolling train where they actually occur. This changes the temperatures at which intermediate reductions occur, and, as a result, also changes the rolling forces associated with these earlier reductions. These changes may adversely affect both metallurgical properties and strip flatness. A second, practical problem is that errors in the predictive calculations, due for example to changes in cooling spray effectiveness, are not known unless sensed by a downstream pyrometer, which is subject to the previously described delay problems.
Other "predictive" approaches are possible, but they all suffer from the drawback of not being able to accurately account for all situations which will be encountered during operation of a hot strip mill. In short, predictive approaches have serious shortcomings, but no completely acceptable adaptive control system exists either.